1. Field of the Invention
The present invention is generally related to photoelectric sensors and, more particularly, to photoelectric sensors which comprise a housing structure with a circular polarizing device attached thereto.
2. Description of the Prior Art
Linear polarizing filters are well known to those skilled in the art. They possess special properties for selectively absorbing light vibrations in certain predefined planes. Unpolarized light comprise a complex mixture of vibrations lying in all possible directions transverse to a line of travel. When the unpolarized light is passed through a linear polarizer, its vibrations become confined to a single linear plane and the light is then considered to be polarized. This linearly polarized light can be modified to suppress unwanted reflections and to eliminate glare for a variety of applications.
Circular polarizing filters are also known to those skilled in the art. Circular polarizers typically comprise a linear polarizer combined with a quarterwave retarder whose slow and fast axes are disposed at a 45 degree angle to the axis of the linear polarizer. Unpolarized light passing through the linear polarizer becomes polarized at a 45 degree angle to the axis of the retarder. When this polarized light ray passes through the retarder, its vibration direction is made to move in a helical pattern. After the light ray is reflected from a specular surface, the sense of rotation of the vibration reverses. This rotation is stopped in the return path through the retarder. The light ray is now linearly polarized in a plane 90 degrees to its original polarization plane and is absorbed by the linearly polarized component of the circular polarizer. Certain polarizers are also particularly configured to transmit only infrared radiation. They absorb visible radiation and transmit infrared radiation. These types of filters can be used in association with photoelectric sensors which must operate in environments with high levels of visible radiation.
U.S. Pat. No. 5,149,962, which issued to Maurice on Sep. 22, 1992, describes a proximity detector which uses the Faraday effect and bidirectional transmission. The proximity detector uses the Faraday effect to detect position changes of a target. The proximity detector includes a permanent magnet having a magnetic field that changes as the target moves to or from a predetermined position. A self-referencing optical sensor is provided that detects the changed magnetic field. The optical sensor includes a magneto-optic material positioned near a node of the magnet. Means are also provided to transmit polarized interrogation light along an optical path that includes the magneto-optic material. The optical path is bidirectionally addressed and is nonreciprocal. The optical sensor transduces the changed magnetic field into a modulation of the interrogation light's intensity. Means are provided to detect variations in the interrogation light intensity and to determine a ration such as the difference over the sum ratio for two interrogation light samples that travel the optic path in opposite directions.
U.S. Pat. No. 4,859,062, which issued to Thurn on Aug. 22, 1989, describes an optoelectrical measuring system and apparatus. The invention relates to measuring the surface roughness of a sample material. A beam of light is directed onto the surface and the scattered light distribution is made using a detector array. Either the average deviation or the second moment of the scattered light distribution is then determined and used as measures of the surface roughness.
U.S. Pat. No. 4,838,683, which issued to Ichihashi et al on Jun. 13, 1989, discloses an ophthalmic measuring method and apparatus. According to this invention, the method and apparatus are provided for the purpose of measuring the state of microparticles in the aqueous chamber of an eye to be examined which is irradiated with a laser beam to detect the scattering characteristics in the aqueous chamber of the eye. The laser beam with a predetermined wavelength is projected onto the aqueous chamber of the eye. Light scattered from the aqueous chamber is received with or without a linear polarizer and evaluated in terms of the changes between the scattering characteristics with or without the linear polarizer in order to determine the number or concentration of the microparticles present in the aqueous chamber for the purpose of detecting ophthalmic diseases.
U.S. Pat. No. 4,786,802, which issued to Yoshii on Nov. 22, 1988, discloses an apparatus for measuring photoelasticity. The device permits the control of mechanical stress applied to an elastic body by visualizing phase differences of polarized light transmitted by the elastic body. In such a prior art apparatus, a quarter wavelength plate was used in order to obtain circularly polarized light. However, the precision of the circularly polarized light is worsened when it works in a wide wavelength region. To the contrary, in an apparatus according to this disclosed device, circularly polarized light is obtained by means of Fresnel's rhombic body. As the result of this device, circularly polarized light can be obtained for a wide wavelength region from the visible region to the near infrared region and control of products including thin films and semiconductor substrates can be effected by visualizing mechanical stress therein.
U.S. Pat. No. 5,115,129, which issued to Johnson on May 19, 1992, discloses a photoelectric device with a lens formed in its housing. The photoelectric device is provided with a one piece housing having a first end which is light transmissive and liquid impermeable. An opening is provided at the other end of the tubular structure for insertion of photoelectric and electronic components into the cavity of the housing structure. A cover is used to seal an opening at the second end. The cover is ultrasonically welded to the second end of the tubular structure and a cable means is provided to permit electrical current to pass through the cover at the second end of the housing structure. The first end of the tubular structure, which is transmissive to visible, infrared or ultraviolet light can be formed into one or more lens, depending on the application of the photoelectric device. Threads are provided in the outer cylindrical surface of the housing to aid in attaching the photoelectric device to brackets with the use of threaded nuts.
U.S. Pat. application Ser. No. 07/842,691 (M10-14748) which was filed on Feb. 27, 1992 by Klima and assigned to the assignee of the present application, discloses a photoelectric sensor with a droplet resistant face. The sensor face is generally flat with a groove formed therein to cause droplets to move away from a central portion of the face where they could otherwise adversely interfere with the operation of the photoelectric sensor. The groove provides a discontinuity in the light transmissive face between the locations of a light source and a light sensitive component.